1. Field of the Invention
The present invention relates to an imaging optical system and an image reading apparatus using the imaging optical system. In particular, the present invention is suitable for reading a monochrome image or a color image using a line sensor of an image scanner, a digital copying machine, or the like in which various aberrations are corrected in a balanced manner to perform image reading with high resolution.
2. Related Background Art
Up to now, various image reading apparatuses have been proposed to read image information on a surface of an original. According to the proposed image reading apparatuses, a line sensor in which a plurality of light receiving elements are arranged in a main scanning direction is used to image the image information on a surface of the line sensor (CCD). The original and the line sensor are moved relative to each other in a sub-scanning direction to read the image information on the surface of the original or the like based on output signals obtained by the line sensor.
FIG. 7 is a schematic view showing a conventional image reading apparatus of a carriage integral type scanning system. In FIG. 7, an original 87 placed on an original table glass 82 is directly illuminated with a light beam emitted from an illumination light source 81. A light beam reflected on the original 87 travels through a first return mirror 83a, a second return mirror 83b, and a third return mirror 83c in this order, the light beam having its optical path bent in an inner portion of a carriage 86. Then, the light beam is imaged on a surface of a line sensor 85 by an imaging lens (imaging optical system) 84.
After that, the carriage 86 is moved in a direction indicated by an arrow A (sub-scanning direction) shown in FIG. 7 by a sub-scanning motor 88 to read the image information on the original 87. The line sensor 85 shown in FIG. 7 is composed of a plurality of light receiving elements arranged in a one-dimensional direction (main scanning direction).
FIG. 8 is an explanatory view showing a fundamental structure of a reading optical system of the image reading optical system shown in FIG. 7.
In FIG. 8, the reading optical system includes the imaging optical system 84 and the line sensor 85. The line sensor 85 is composed of line sensors 85R, 85G, and 85B for reading color information of R (red), G (green), and B (blue). Reading areas 87R, 87G, and 87B are set on the surface of the original 87 corresponding to the line sensors 85R, 85G, and 85B.
When the surface of the original 87 is scanned, the same location can be read in different colors at a certain time interval. In the case where the imaging optical system 84 includes an ordinary refracting system in the above-mentioned structure, longitudinal chromatic aberration and lateral chromatic aberration occur. Therefore, defocus or positional misregistration occurs in line images to be formed on the line sensors 85B and 85R as compared with the case of the line sensor 85G serving as a reference sensor. Thus, when the respective color images are superimposed for reproduction, color bleeding or misregistration is conspicuous in a resultant image. That is, when high-aperture performance and high-resolution performance are required, such requirements cannot be satisfied.
On the other hand, according to recent proposals, even in the case of a non-coaxial optical system, it is possible to construct an optical system whose aberrations are sufficiently corrected by introducing the concept of a reference axis to make constituent surfaces thereof asymmetrical and aspherical (see Japanese Patent Application Laid-Open No. H09-005650, Japanese Patent Application Laid-Open No. H08-292371, and Japanese Patent Application Laid-Open No. H08-292372). An example of a designing method of the optical system is disclosed in Japanese Patent Application Laid-Open H09-005650 and design examples thereof are disclosed in Japanese Patent Application Laid-Open H08-292371 and Japanese Patent Application Laid-Open No. H08-292372.
Such a non-coaxial optical system is called an off-axial optical system. The off-axial optical system is defined as an optical system including a curved surface (an off-axial curved surface) in which, when a reference axis is set along a light beam passing through the center of an image and the center of a pupil, a surface normal to a constituent surface at an intersection with the reference axis is not on the reference axis. At this time, the reference axis becomes a bent shape.
In the off-axial optical system, a constituent surface thereof normally becomes non-coaxial and no eclipse occurs even on a reflective surface thereof, so an optical system using the reflective surface is easy to construct. The off-axial optical system also has advantages that an optical path can be relatively freely drawn and that an integral type optical system is easy to produce by using a method of integrally molding constituent surfaces.
There has been disclosed an imaging optical system for image reading to which such a technique is adopted (see Japanese Patent Application Laid-Open No. 2002-335375). When the disclosed technique is used, an off-axial optical system including five or six reflective surfaces (off-axial reflective surfaces) in which there is no chromatic aberration and other aberrations are sufficiently corrected is achieved in an image reading apparatus. In addition to this, the image forming optical apparatus is reduced in size, so an optical system suitable for a carriage integral type is provided in the example.
It has been disclosed an imaging optical system for image reading to which the same technique is adopted (see Japanese Patent Application Laid-Open No. 2003-057549). In the example, an off-axial optical system including three reflective surfaces (off-axial reflective surfaces) is disclosed, which has an optical path length sufficient for an application to a 2:1 mirror scanning type scanner.
There has been disclosed an invention as to positional deviation of an imaging plane resulting from a change in temperature occurring in the case where a resin material is used to simplify the off-axial optical system (to reduce a cost) (see Japanese Patent Application Laid-Open No. 2003-287683).
On the other hand, in the case of such a reflection type off-axial optical system, it is difficult to maintain preferable optical performances with each of surfaces thereof formed in a spherical shape. However, when at least one surface is formed to be an aspherical surface (free surface) which is rotational asymmetrical, the preferable optical performances can be achieved.
It has been generally known that an optical system composed of refractive surfaces is sensitive to decentering. When a reflection type optical element having the aspherical surface (free surface) which is rotational asymmetrical is to be incorporated in the off-axial optical system, it is required that, in addition to the precision of a normal spherical reflective surface, a member for holding the normal spherical reflective surface and the reflection type optical element be formed with high precision.
In the case where the off-axial optical reflective surface is a reflective surface having a free surface shape, production of the off-axial optical system made of normal glass makes its manufacturing process complicated, which increases a cost thereof. In order to deal with such a problem, for example, the off-axial optical system may be made of plastic such as polycarbonate, acrylic, or polyolefin. However, in addition to a problem with the misregistration of an imaging position resulting from a change of environment, particularly, a change in temperature as described in Japanese Patent Application Laid-Open No. 2003-287683, there occurs another problem in that the optical element (off-axial optical element) deforms under its own weight to deviate the imaging position.
A resin material, which is normally used as an optical material, has a flexural modulus much smaller than that of a glass material, so the resin material easily deforms. When strength is merely provided to an optical element so as to prevent the optical element from deforming under its own weight, the optical element is made large and thickened. Thus, a time for molding the optical element lengthens or the number of cavities reduces, which leads to a problem in that manufacturing becomes difficult (thereby increasing a cost).